i think they should GW2 system, each time you craft something the cast speed increases slightly, so when you have 200 bars to smelt you don't have to stand there for 10mins.
Yea I had someone smelt 1,6k bars recently and with trading it took three Hours. And some Months ago I did couple thousands Shoulder Enchants with 1-Button Macro spam. Did it with little WoW-Window so could watch Youtoob. Finished the whole Playthrough of Starcraft 2 Heart of the SwarmSolocampaign before finished Milling. Made some interrupts for a few Minutes whenever a Warbringer spawned but still...
Spending dozens of Hours(!) to convert Ore or Herbs into higher quality crafting Materials is not a good game experience. For Example, I want to sell ten of new 5.4 Sky Golem Mounts. To craft them, one needs over 26 Hours, more than an actual day, doing nothing but smelting.
I've never had smelting problems after I switched off my crafting addons. Before then, regularly. I'd put 60 stacks of ore to smelt, only to come back after 15 minutes and find the work had stopped after 5 stacks, and I'd STILL need to sit there for 15 minutes waiting for the bunch to smelt. >.<